Cardiomyopathy-linked Mutations Research Articles

Assessment of dynamic cardiac repolarization and contractility in patients with hypertrophic cardiomyopathy.

Arrhythmia mechanisms in hypertrophic cardiomyopathy remain uncertain. Preclinical models suggest hypertrophic cardiomyopathy-linked mutations perturb sarcomere length-dependent activation, alter cardiac repolarization in rate-dependent fashion and potentiate triggered electrical activity. This study was designed to assess rate-dependence of clinical surrogates of contractility and repolarization in humans with hypertrophic cardiomyopathy. All participants had a cardiac implantable device capable of atrial pacing. Cases had clinical diagnosis of hypertrophic cardiomyopathy, controls were age-matched. Continuous electrocardiogram and blood pressure were recorded during and immediately after 30 second pacing trains delivered at increasing rates. Nine hypertrophic cardiomyopathy patients and 10 controls were enrolled (47% female, median 55 years), with similar baseline QRS duration, QT interval and blood pressure. Median septal thickness in hypertrophic cardiomyopathy patients was 18mm; 33% of hypertrophic cardiomyopathy patients had peak sub-aortic velocity >50mmHg. Ventricular ectopy occurred during or immediately after pacing trains in 4/9 hypertrophic cardiomyopathy patients and 0/10 controls (P = 0.03). During delivery of steady rate pacing across a range of cycle lengths, the QT-RR relationship was not statistically different between HCM and control groups; no differences were seen in subgroup analysis of patients with or without intact AV node conduction. Similarly, there was no difference between groups in the QT interval of the first post-pause recovery beat after pacing trains. No statistically significant differences were seen in surrogate measures for cardiac contractility. Rapid pacing trains triggered ventricular ectopy in hypertrophic cardiomyopathy patients, but not controls. This finding aligns with pre-clinical descriptions of excessive cardiomyocyte calcium loading during rapid pacing, increased post-pause sarcoplasmic reticulum calcium release, and subsequent calcium-triggered activity. Normal contractility at all diastolic intervals argues against clinical significance of altered length-dependent myofilament activation.

Computational and Experimental Investigation of Cardiac Troponin T R173Q, R173W and Δ160E Mutation Specific Correlates to Disease

Mutations in cardiac troponin T (cTnT), a component of the cardiac thin filament (cTF), are known to cause changes in protein structure and dynamics leading to pathological cardiac remodeling observed in patients with hypertrophic (HCM) and dilated (DCM) cardiomyopathies. In this complex system, mutations in cTnT separated by only a few amino acids can cause divergent cardiomyopathies, highlighting the importance of primary structural and dynamic “triggers” that lead to disease. To begin to correlate genotype to phenotype, we investigated three cardiomyopathy-linked mutations in the cTnT linker region: DCM-associated mutations R173W and R173Q, and HCM-associated mutation Δ160E. We coupled computational predictions with in-vitro biophysical measurementsto better understand the divergent disease mechanisms. We hypothesize that R173Q/W and Δ160E cause differential structural changes in the cTF linker domain leading to atomic and molecular level changes that initiate pathogenic remodeling. Differential scanning calorimetry on all 3 mutations showed an increased thermal stability required to unfold the Tm:Tn complex from actin, suggesting altered protein interactions. To obtain further resolution, molecular dynamics (MD) simulations and time-resolved fluorescence resonance energy transfer (TR-FRET) studies were employed. Previous studies showed that Δ160E caused a decreased distance between the linker region of cTnT and the tropomyosin C-terminus, resulting in decreased linker flexibility. In contrast, MD simulations of the TF containing R173W/Q show an increase distance between the linker region of cTnT and the C-terminus of tropomyosin and actin 374. Further clarification of these data will be provided by ongoing TR-FRET studies to confirm structural distance changes in the liker region in fully reconstituted cTF. This iterative process will allow us to build predictive tools to test cTF mutations associated with these cardiomyopathies and eventually to design and implement more precise therapeutic interventions.

Effects of cardiomyopathy-linked mutations K15N and R21H in tropomyosin on thin-filament regulation and pointed-end dynamics.

Missense mutations K15N and R21H in striated muscle tropomyosin are linked to dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM), respectively. Tropomyosin, together with the troponin complex, regulates muscle contraction and, along with tropomodulin and leiomodin, controls the uniform thin-filament lengths crucial for normal sarcomere structure and function. We used Förster resonance energy transfer to study effects of the tropomyosin mutations on the structure and kinetics of the cardiac troponin core domain associated with the Ca2+-dependent regulation of cardiac thin filaments. We found that the K15N mutation desensitizes thin filaments to Ca2+ and slows the kinetics of structural changes in troponin induced by Ca2+ dissociation from troponin, while the R21H mutation has almost no effect on these parameters. Expression of the K15N mutant in cardiomyocytes decreases leiomodin’s thin-filament pointed-end assembly but does not affect tropomodulin’s assembly at the pointed end. Our in vitro assays show that the R21H mutation causes a twofold decrease in tropomyosin’s affinity for F-actin and affects leiomodin’s function. We suggest that the K15N mutation causes DCM by altering Ca2+-dependent thin-filament regulation and that one of the possible HCM-causing mechanisms by the R21H mutation is through alteration of leiomodin’s function.

Molecular defects in cardiac myofilament Ca2+- regulation due to cardiomyopathy-linked mutations can be reversed by small molecules binding to troponin

Molecular defects in cardiac myofilament Ca2+- regulation due to cardiomyopathy-linked mutations can be reversed by small molecules binding to troponin

Molecular Defects in Cardiac Myofilament Ca2+-Regulation Due to Cardiomyopathy-Linked Mutations Can Be Reversed by Small Molecules Binding to Troponin.

The inherited cardiomyopathies, hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are relatively common, potentially life-threatening and currently untreatable. Mutations are often in the contractile proteins of cardiac muscle and cause abnormal Ca2+ regulation via troponin. HCM is usually linked to higher myofilament Ca2+-sensitivity whilst in both HCM and DCM mutant tissue there is often an uncoupling of the relationship between troponin I (TnI) phosphorylation by PKA and modulation of myofilament Ca2+-sensitivity, essential for normal responses to adrenaline. The adrenergic response is blunted, and this may predispose the heart to failure under stress. At present there are no compounds or interventions that can prevent or treat sarcomere cardiomyopathies. There is a need for novel therapies that act at a more fundamental level to affect the disease process. We demonstrated that epigallocatechin-3 gallate (EGCG) was found to be capable of restoring the coupled relationship between Ca2+-sensitivity and TnI phosphorylation in mutant thin filaments to normal in vitro, independent of the mutation (15 mutations tested). We have labeled this property “re-coupling.” The action of EGCG in vitro to reverse the abnormality caused by myopathic mutations would appear to be an ideal pharmaceutical profile for treatment of inherited HCM and DCM but EGCG is known to be promiscuous in vivo and is thus unsuitable as a therapeutic drug. We therefore investigated whether other structurally related compounds can re-couple myofilaments without these off-target effects. We used the quantitative in vitro motility assay to screen 40 compounds, related to C-terminal Hsp90 inhibitors, and found 23 that can re-couple mutant myofilaments. There is no correlation between re-couplers and Hsp90 inhibitors. The Ca2+-sensitivity shift due to TnI phosphorylation was restored to 2.2 ± 0.01-fold (n = 19) compared to 2.0 ± 0.24-fold (n = 7) in wild-type thin filaments. Many of these compounds were either pure re-couplers or pure desensitizers, indicating these properties are independent; moreover, re-coupling ability could be lost with small changes of compound structure, indicating the possibility of specificity. Small molecules that can re-couple may have therapeutic potential.HIGHLIGHTS - Inherited cardiomyopathies are common diseases that are currently untreatable at a fundamental level and therefore finding a small molecule treatment is highly desirable.- We have identified a molecular level dysfunction common to nearly all mutations: uncoupling of the relationship between troponin I phosphorylation and modulation of myofilament Ca2+-sensitivity, essential for normal responses to adrenaline.- We have identified a new class of drugs that are capable of both reducing Ca2+-sensitivity and/or recouping the relationship between troponin I phosphorylation and Ca2+-sensitivity.- The re-coupling phenomenon can be explained on the basis of a single mechanism that is testable.- Measurements with a wide range of small molecules of varying structures can indicate the critical molecular features required for recoupling and allows the prediction of other potential re-couplers.

Comparable Calcium Handling and Contractility in Human IPSC Cardiomyocyte Models of Three Different Hypertrophic Cardiomyopathy-Linked Mutations

Comparable Calcium Handling and Contractility in Human IPSC Cardiomyocyte Models of Three Different Hypertrophic Cardiomyopathy-Linked Mutations

Structural destabilization of tropomyosin induced by the cardiomyopathy-linked mutation R21H.

The missense mutation R21H in striated muscle tropomyosin is associated with hypertrophic cardiomyopathy, a genetic cardiac disease and a leading cause of sudden cardiac death in young people. Tropomyosin adopts conformation of a coiled coil which is critical for regulation of muscle contraction. In this study, we investigated the effects of the R21H mutation on the coiled-coil structure of tropomyosin and its interactions with its binding partners, tropomodulin and leiomodin. Using circular dichroism and isothermal titration calorimetry, we found that the mutation profoundly destabilized the structural integrity of αTM1a1-28 Zip, a chimeric peptide containing the first 28 residues of tropomyosin. The mutated αTM1a1-28 Zip was still able to interact with tropomodulin and leiomodin. However, the mutation resulted in a ∼30-fold decrease of αTM1a1-28 Zip's binding affinity to leiomodin. We used a crystal structure of αTM1a1-28 Zip that we solved at 1.5 Å resolution to study the mutation's effect in silico by means of molecular dynamics simulation. The simulation data indicated that while the mutation disrupted αTM1a1-28 Zip's coiled-coil structure, most notably from residue Ala18 to residue His31, it may not affect the N-terminal end of tropomyosin. The drastic decrease of αTM1a1-28 Zip's affinity to leiomodin caused by the mutation may lead to changes in the dynamics at the pointed end of thin filaments. Therefore, the R21H mutation is likely interfering with the regulation of the normal thin filament length essential for proper muscle contraction.

Hypertrophic cardiomyopathy mutation in cardiac troponin T (R95H) attenuates length-dependent activation in guinea pig cardiac muscle fibers.

The central region of cardiac troponin T (TnT) is important for modulating the dynamics of muscle length-mediated cross-bridge recruitment. Therefore, hypertrophic cardiomyopathy mutations in the central region may affect cross-bridge recruitment dynamics to alter myofilament Ca2+ sensitivity and length-dependent activation of cardiac myofilaments. Given the importance of the central region of TnT for cardiac contractile dynamics, we studied if hypertrophic cardiomyopathy-linked mutation (TnTR94H)-induced effects on contractile function would be differently modulated by sarcomere length (SL). Recombinant wild-type TnT (TnTWT) and the guinea pig analog of the human R94H mutation (TnTR95H) were reconstituted into detergent-skinned cardiac muscle fibers from guinea pigs. Steady-state and dynamic contractile measurements were made at short and long SLs (1.9 and 2.3 µm, respectively). Our results demonstrated that TnTR95H increased pCa50 (-log of free Ca2+ concentration) to a greater extent at short SL; TnTR95H increased pCa50 by 0.11 pCa units at short SL and 0.07 pCa units at long SL. The increase in pCa50 associated with an increase in SL from 1.9 to 2.3 µm (ΔpCa50) was attenuated nearly twofold in TnTR95H fibers; ΔpCa50 was 0.09 pCa units for TnTWT fibers but only 0.05 pCa units for TnTR95H fibers. The SL dependency of rate constants of cross-bridge distortion dynamics and tension redevelopment was also blunted by TnTR95H Collectively, our observations on the SL dependency of pCa50 and rate constants of cross-bridge distortion dynamics and tension redevelopment suggest that mechanisms underlying the length-dependent activation cardiac myofilaments are attenuated by TnTR95HNEW & NOTEWORTHY Mutant cardiac troponin T (TnTR95H) differently affects myofilament Ca2+ sensitivity at short and long sarcomere length, indicating that mechanisms underlying length-dependent activation are altered by TnTR95H TnTR95H enhances myofilament Ca2+ sensitivity to a greater extent at short sarcomere length, thus attenuating the length-dependent increase in myofilament Ca2+ sensitivity.

Structural Destabilization of Tropomyosin Induced by a Cardiomyopathy-Linked Mutation

In striated muscle, thin filaments, composed of F-actin, and thick filaments constitute the basic contractile units called sarcomeres. Tropomyosin (Tpm), a two-stranded coiled-coil protein, binds along the actin filaments through head-to-tail polymerization, protects and stabilizes the thin filaments. The N-terminus of Tpm orients toward the pointed end of thin filament, where it interacts with pointed end-binding proteins such as tropomodulin (Tmod) and leiomodin (Lmod) to maintain the uniform filament length critical for proper sarcomeric functions. Recently, a hypertrophic cardiomyopathy-associated mutation, R21H, has been identified in striated muscle Tpm (Tpm1.1) with molecular mechanism of perturbing muscular function unknown. We designed, expressed, and purified the Tpm chimeric peptide αTM1a1-28Zip. The peptide consists of 28 N-terminal residues of Tpm1.1 followed by 18 C-terminal residues of the GCN4 leucine zipper domain. The peptide was crystallized and its structure was solved. To study how this mutation affects Tpm1.1, we introduced the mutation R21H in the peptide. An effect of the mutation was studied in silico using molecular dynamics simulation (MDS) and in vitro by circular dichroism (CD). Temperature measurements using CD were conducted to characterize the effect of the mutation R21H on thermal stability of the αTM1a1-28Zip peptide alone and its complexes with Tmod and Lmod fragments containing Tpm-binding sites. CD data showed that the mutation R21H caused a significant decrease in the helical content and structural stability of αTM1a1-28Zip. Complexes formed between the αTM1a1-28Zip[R21H] peptide and Tmod or Lmod fragments were less stable than those formed with wild-type αTM1a1-28Zip. All CD data were in agreement with MDS results which showed that the mutation R21H significantly altered the coiled-coil structure of αTM1a1-28Zip. We suggest that the mutation R21H destabilizes Tpm structure by disrupting local salt bridges formed between residues Arg21 and Glu26 on opposite strands.

Exploration of pathomechanisms triggered by a single-nucleotide polymorphism in titin's I-band: the cardiomyopathy-linked mutation T2580I.

Missense single-nucleotide polymorphisms (mSNPs) in titin are emerging as a main causative factor of heart failure. However, distinguishing between benign and disease-causing mSNPs is a substantial challenge. Here, we research the question of whether a single mSNP in a generic domain of titin can affect heart function as a whole and, if so, how. For this, we studied the mSNP T2850I, seemingly linked to arrhythmogenic right ventricular cardiomyopathy (ARVC). We used structural biology, computational simulations and transgenic muscle in vivo methods to track the effect of the mutation from the molecular to the organismal level. The data show that the T2850I exchange is compatible with the domain three-dimensional fold, but that it strongly destabilizes it. Further, it induces a change in the conformational dynamics of the titin chain that alters its reactivity, causing the formation of aberrant interactions in the sarcomere. Echocardiography of knock-in mice indicated a mild diastolic dysfunction arising from increased myocardial stiffness. In conclusion, our data provide evidence that single mSNPs in titin's I-band can alter overall muscle behaviour. Our suggested mechanisms of disease are the development of non-native sarcomeric interactions and titin instability leading to a reduced I-band compliance. However, understanding the T2850I-induced ARVC pathology mechanistically remains a complex problem and will require a deeper understanding of the sarcomeric context of the titin region affected.

Increased Regional Pannexin1 (Px1) Channel Expression linked to Focal Ischemia is Pro‐Arrhythmic During Stress

Mice with the hypertrophic cardiomyopathy‐linked TnT‐I79N mutation develop arrhythmias during physiological stress. In isolated hearts these arrhythmias are linked to regional ischemia (no decrease in global coronary flow) which causes regional hypoxia and cellular energy deprivation and results in regional conduction velocity slowing and increased arrhythmia susceptibility (84.7% vs 14.3% in control).To test if regional hypoxia/ischemia also occurred spontaneously in vivo in TnT‐I79N mice, we measured mRNA expression of genes up‐regulated by hypoxia (GLUT1, TUBB3, Px1) and found increased expression (0.3fold, 3fold, 10fold respectively). To determine the spatial distribution of Px1‐hypoxia marker we first subjected Px1 ‐βGal reporter mice to coronary ligation and demonstrate that Xgal staining is limited to the perfusion bed of the ligated artery. Then we crossed the TnT‐I79N with the Px1 reporter mice and found that Xgal staining increased in a spatially non‐uniform “patchy” distribution, consistent with focal ischemia. Px1 channels have been implicated in the regulation of vascular perfusion during hypoxia. Strikingly, ablation of Px1 in TnT‐I79N mice reduced the arrhythmia incidence in isolated hearts (to 36.4%) and prevented the focal energy deprivation during stress.Although the physiological role for the increase in Px1 after ischemia remains unclear, Px1 contributes to the increased arrhythmia susceptibility in TnT‐I79N mice. Hence, our data suggest that Px1 may be involved in the regulation of local coronary perfusion and is maladaptive during stress.

Impact of familial hypertrophic cardiomyopathy-linked mutations in the NH2 terminus of the RLC on β-myosin cross-bridge mechanics.

Familial hypertrophic cardiomyopathy (HCM) is associated with mutations in sarcomeric proteins, including the myosin regulatory light chain (RLC). Here we studied the impact of three HCM mutations located in the NH2 terminus of the RLC on the molecular mechanism of β-myosin heavy chain (MHC) cross-bridge mechanics using the in vitro motility assay. To generate mutant β-myosin, native RLC was depleted from porcine cardiac MHC and reconstituted with mutant (A13T, F18L, and E22K) or wild-type (WT) human cardiac RLC. We characterized the mutant myosin force and motion generation capability in the presence of a frictional load. Compared with WT, all three mutants exhibited reductions in maximal actin filament velocity when tested under low or no frictional load. The actin-activated ATPase showed no significant difference between WT and HCM-mutant-reconstituted myosins. The decrease in velocity has been attributed to a significantly increased duty cycle, as was measured by the dependence of actin sliding velocity on myosin surface density, for all three mutant myosins. These results demonstrate a mutation-induced alteration in acto-myosin interactions that may contribute to the pathogenesis of HCM.

Cardiac Muscle Activation Blunted by a Mutation to the Regulatory Component, Troponin T

The striated muscle thin filament comprises actin, tropomyosin, and troponin. The Tn complex consists of three subunits, troponin C (TnC), troponin I (TnI), and troponin T (TnT). TnT may serve as a bridge between the Ca(2+) sensor (TnC) and the actin filament. In the short helix preceding the IT-arm region, H1(T2), there are known dilated cardiomyopathy-linked mutations (among them R205L). Thus we hypothesized that there is an element in this short helix that plays an important role in regulating the muscle contraction, especially in Ca(2+) activation. We mutated Arg-205 and several other amino acid residues within and near the H1(T2) helix. Utilizing an alanine replacement method to compare the effects of the mutations, the biochemical and mechanical impact on the actomyosin interaction was assessed by solution ATPase activity assay, an in vitro motility assay, and Ca(2+) binding measurements. Ca(2+) activation was markedly impaired by a point mutation of the highly conserved basic residue R205A, residing in the short helix H1(T2) of cTnT, whereas the mutations to nearby residues exhibited little effect on function. Interestingly, rigor activation was unchanged between the wild type and R205A TnT. In addition to the reduction in Ca(2+) sensitivity observed in Ca(2+) binding to the thin filament, myosin S1-ADP binding to the thin filament was significantly affected by the same mutation, which was also supported by a series of S1 concentration-dependent ATPase assays. These suggest that the R205A mutation alters function through reduction in the nature of cooperative binding of S1.

Significance of Troponin Dynamics for Ca2+-Mediated Regulation of Contraction and Inherited Cardiomyopathy

Ca2+ dissociation from troponin causes cessation of muscle contraction, by incompletely understood structural mechanisms. To investigate this process, regulatory site Ca2+ binding in subunit TnC's NH2-lobe was abolished by mutagenesis, and effects on cardiac troponin dynamics were mapped by hydrogen-deuterium exchange-MS (HDX). The findings demonstrate the inter-relationships among troponin's detailed dynamics, troponin's regulatory actions, and the pathogenesis of cardiomyopathy linked to troponin mutations. Ca2+ slowed HDX up to two orders of magnitude within the NH2-lobe and the NH2-lobe-associated TnI switch helix, implying that Ca2+ greatly stabilizes this troponin regulatory region. HDX of the TnI COOH-terminus indicated that its known role in regulation involves a partially folded rather than unfolded structure in the absence of Ca2+ and actin. Ca2+-triggered stabilization extended beyond the known direct regulatory regions: to the start of the nearby TnI helix 1, and to the COOH- terminus of the TnT-TnI coiled-coil. Ca2+ destabilized rather than stabilized specific TnI segments within the coiled-coil, and destabilized a region not previously implicated in Ca2+-mediated regulation: the coiled-coil's NH2-terminal base plus the preceding TnI loop with which the base interacts. Cardiomyopathy-linked mutations clustered almost entirely within very dynamic regions of troponin, and many sites were Ca2+ sensitive. Overall, the findings demonstrate highly selective effects of regulatory site Ca2+, including opposite changes in protein dynamics at opposite ends of the troponin core domain. Ca2+ release triggers an intra-molecular switching mechanism that propagates extensively within the extended troponin structure, suggests specific movements of the TnI inhibitory regions, and prominently involves troponin's dynamic features.

Significance of Troponin Dynamics for Ca2+-mediated Regulation of Contraction and Inherited Cardiomyopathy

Ca(2+) dissociation from troponin causes cessation of muscle contraction by incompletely understood structural mechanisms. To investigate this process, regulatory site Ca(2+) binding in the NH(2)-lobe of subunit troponin C (TnC) was abolished by mutagenesis, and effects on cardiac troponin dynamics were mapped by hydrogen-deuterium exchange (HDX)-MS. The findings demonstrate the interrelationships among troponin's detailed dynamics, troponin's regulatory actions, and the pathogenesis of cardiomyopathy linked to troponin mutations. Ca(2+) slowed HDX up to 2 orders of magnitude within the NH(2)-lobe and the NH(2)-lobe-associated TnI switch helix, implying that Ca(2+) greatly stabilizes this troponin regulatory region. HDX of the TnI COOH terminus indicated that its known role in regulation involves a partially folded rather than unfolded structure in the absence of Ca(2+) and actin. Ca(2+)-triggered stabilization extended beyond the known direct regulatory regions: to the start of the nearby TnI helix 1 and to the COOH terminus of the TnT-TnI coiled-coil. Ca(2+) destabilized rather than stabilized specific TnI segments within the coiled-coil and destabilized a region not previously implicated in Ca(2+)-mediated regulation: the coiled-coil's NH(2)-terminal base plus the preceding TnI loop with which the base interacts. Cardiomyopathy-linked mutations clustered almost entirely within influentially dynamic regions of troponin, and many sites were Ca(2+)-sensitive. Overall, the findings demonstrate highly selective effects of regulatory site Ca(2+), including opposite changes in protein dynamics at opposite ends of the troponin core domain. Ca(2+) release triggers an intramolecular switching mechanism that propagates extensively within the extended troponin structure, suggests specific movements of the TnI inhibitory regions, and prominently involves troponin's dynamic features.

King of hearts: a splicing factor rules cardiac proteins

Alternative splicing ensures the expression of functionally diverse proteins from individual genes; however, aberrant mRNA splicing is associated with various conditions, including heart disease. A recent study provides new mechanistic insights into heart failure by showing that a human cardiomyopathy-linked mutation in a cardiac splice factor affects post-transcriptional regulation, causing the expression of anomalous isoforms of a whole network of cardiac proteins (pages 766-773).

Structure and Interactions of Myosin-binding Protein C Domain C0: CARDIAC-SPECIFIC REGULATION OF MYOSIN AT ITS NECK?

Myosin-binding protein C (MyBP-C) is a multidomain protein present in the thick filaments of striated muscles and is involved in both sarcomere formation and contraction regulation. The latter function is believed to be located at the N terminus, which is close to the motor domain of myosin. The cardiac isoform of MyBP-C is linked to hypertrophic cardiomyopathy. Here, we use NMR spectroscopy and biophysical and biochemical assays to study the three-dimensional structure and interactions of the cardiac-specific Ig-like domain C0, a part of cardiac MyBP-C of which little is known. The structure confirmed that C0 is a member of the IgI class of proteins, showing many of the characteristic features of this fold. Moreover, we identify a novel interaction between C0 and the regulatory light chain of myosin, thus placing the N terminus of the protein in proximity to the motor domain of myosin. This novel interaction is disrupted by several cardiomyopathy-linked mutations in the MYBPC3 gene. These results provide new insights into how cardiac MyBP-C incorporates in the sarcomere and how it can contribute to the regulation of muscle contraction.

Hypertrophic Cardiomyopathy-Linked Mutation D145E Drastically Alters Calcium Binding by the C-Domain of Cardiac Troponin C

The role of the C-domain sites of cardiac troponin C in the modulation of the calcium signal remains unclear. In this study, we investigated the effects of hypertrophic cardiomyopathy-linked mutations A8V, E134D, and D145E in cardiac troponin C on the properties of the C-domain sites. The A8V mutation had essentially no effect on the calcium or magnesium binding properties of the C-domain sites, while the mutation E134D moderately decreased calcium and magnesium binding affinities. On the other hand, the D145E mutation affected cooperative interactions between sites III and IV, significantly reducing the calcium binding affinity of both sites. Binding of the anchoring region of cardiac troponin I (corresponding to residues 34-71) to cardiac troponin C with the D145E mutation was not able to recover normal calcium binding to the C-domain. Experiments utilizing the fluorescent hydrophobic probe bis-ANS suggest that the D145E mutation dramatically reduced the extent of calcium-induced hydrophobic exposure by the C-domain. At high nonphysiological calcium concentration, A8V, E134D, and D145E mutations minimally affected the affinity of cardiac troponin C for the regulatory region of cardiac troponin I (corresponding to residues 128-180). In contrast, at lower physiological calcium concentration, the D145E mutation led to an approximately 8-fold decrease in the affinity of cardiac troponin C for the regulatory region of cardiac troponin I. Our results suggest that calcium binding properties of the C-domain sites might be important for the proper regulatory function of cardiac troponin C.

Ala Scanning of the Inhibitory Region of Cardiac Troponin I

In skeletal and cardiac muscles, troponin (Tn), which resides on the thin filament, senses a change in intracellular Ca(2+) concentration. Tn is composed of TnC, TnI, and TnT. Ca(2+) binding to the regulatory domain of TnC removes the inhibitory effect by TnI on the contraction. The inhibitory region of cardiac TnI spans from residue 138 to 149. Upon Ca(2+) activation, the inhibitory region is believed to be released from actin, thus triggering actin-activation of myosin ATPase. In this study, we created a series of Ala-substitution mutants of cTnI to delineate the functional contribution of each amino acid in the inhibitory region to myofilament regulation. We found that most of the point mutations in the inhibitory region reduced the ATPase activity in the presence of Ca(2+), which suggests the same region also acts as an activator of the ATPase. The thin filaments can also be activated by strong myosin head (S1)-actin interactions. The binding of N-ethylmaleimide-treated myosin subfragment 1 (NEM-S1) to actin filaments mimics such strong interactions. Interestingly, in the absence of Ca(2+) NEM-S1-induced activation of S1 ATPase was significantly less with the thin filaments containing TnI(T144A) than that with the wild-type TnI. However, in the presence of Ca(2+), there was little difference in the activation of ATPase activity between these preparations.

Dissecting the N-terminal Myosin Binding Site of Human Cardiac Myosin-binding Protein C: STRUCTURE AND MYOSIN BINDING OF DOMAIN C2

Myosin-binding protein C (MyBP-C) binds to myosin with two binding sites, one close to the N terminus and the other at the C terminus. Here we present the solution structure of one part of the N-terminal binding site, the third immunoglobulin domain of the cardiac isoform of human MyBP-C (cC2) together with a model of its interaction with myosin. Domain cC2 has the beta-sandwich structure expected from a member of the immunoglobulin fold. The C-terminal part of the structure of cC2 is very closely related to telokin, the myosin binding fragment of myosin light chain kinase. Domain cC2 also contains two cysteines on neighboring strands F and G, which would be able to form a disulfide bridge in a similar position as in telokin. Using NMR spectroscopy and isothermal titration calorimetry we demonstrate that cC2 alone binds to a fragment of myosin, S2Delta, with low affinity (kD = 1.1 mM) but exhibits a highly specific binding site. This consists of the C-terminal surface of the C'CFGA' beta-sheet, which includes Glu(301), a residue mutated to Gln in the disease familial hypertrophic cardiomyopathy. The binding site on S2 was identified by a combination of NMR binding experiments of cC2 with S2Delta containing the cardiomyopathy-linked mutation R870H and molecular modeling. This mutation lowers the binding affinity and changes the arrangement of side chains at the interface. Our model of the cC2-S2Delta complex gives a first glimpse of details of the MyBP-C-myosin interaction. Using this model we suggest that most key interactions are between polar amino acids, explaining why the mutations E301Q in cC2 and R870H in S2Delta could be involved in cardiomyopathy. We expect that this model will stimulate future research to further refine the details of this interaction and their importance for cardiomyopathy.